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Introduction to Classification of Animals

The Kingdom Animalia includes about 1.3 million animal species with great diversity in their living form. They differ from one another with respect to size, external and internal morphology, behaviour, physiology and mode of reproduction which makes difficult for one to study the details of individual animal. To overcome this problem, animals are classified on the basis of their general characters (see Figure 3.1).


An evolutionary tree diagram illustrating the relationship between the groups of animals is as shown in Figure 3.2.

Key characteristics that distinguish the eight groups of invertebrates are as shown in Table 3.1.


 Classification of Animals

  • Asymetrical: It refers to complete absence of symmetry.
  • Symmetrical: The body of the organism which is capable of division by a longitudinal plane into similar halves is referred as symmetrical organism.
  • Radial: It is defined as any plane passing through the central axis that divide the body into two identical halves.
  • Bilateral: A body that can be divided into left and right halves by only one plane is said to be bilateral.
  • Diploblastic:
  1. Cells that are arranged in two embryonic layers, ectoderm and endoderm, are called diploblastic.
  2. Undifferentiated layer, i.e., mesoglea, is present in between two embryonic layers.
  • Triploblastic: Cells that are arranged in three embryonic layers, namely ectoderm, mesoderm and endoderm, are called triploblastic.
  • Acoelomate: These are animals without coelom.
  • Pseudocoelomate: Any of a group in invertebrates with a three-layered body that has a fluid-filled body cavity (pseudocoelom) between the endoderm and the mesoderm (the innermost and middle tissue layers) are referred to as pseudocoelomates.
  • Eucoelomate: These are animals with true body cavity.

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